Using data from three waves of a longitudinal study of IDUs residing in Baltimore City, this study will be the first to assess the stability of IDUs' social networks and the consequences of stability or instability on HIV risk behaviors. An understanding of factors that influence the stability of IDUs' social networks is critical because in addition to offering protection through continuation with injection partners whose HIV status is known, stability of networks may also promote protective behavior through the establishment of risk-reduction norms (e.g., never sharing injection equipment). This study seeks to determine whether there are systematic associations between, on the one hand, characteristics of individual IDUs and their social networks, and on the other hand, the stability of risk relationships and behaviors over time. The specific aims of this study are the following: 1) to describe the stability of the social relationships of this population of IDUs over the last four years; 2) to assess whether IDUs in this population are being selective about with whom they are sharing syringes and other injection equipment; 3) to identify the individual and network characteristics that are associated with stability in social relationships; 4) to develop measures of the strength of social relationships in order to identify the social network indicators that best describe the stability of a relationship; and, 5) to develop a multivariate model of factors that are associated with the stability of IDUs' social networks and consequently risk of HIV exposure.